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Endocannabinoid modulation of dopaminergic motor circuits

1 Faculty of Medicine and Dentistry, Department of Pharmacology, University of the Basque Country, Leioa, Spain

2 Faculty of Pharmacy, Department of Pharmacology, University of the Basque Country, Vitoria-Gasteiz, Spain

There is substantial evidence supporting a role for the endocannabinoid system as a modulator of the dopaminergic activity in the basal ganglia, a forebrain system that integrates cortical information to coordinate motor activity regulating signals. In fact, the administration of plant-derived, synthetic or endogenous cannabinoids produces several effects on motor function. These effects are mediated primarily through the CB1 receptors that are densely located in the dopamine-enriched basal ganglia networks, suggesting that the motor effects of endocannabinoids are due, at least in part, to modulation of dopaminergic transmission. On the other hand, there are profound changes in CB1 receptor cannabinoid signaling in the basal ganglia circuits after dopamine depletion (as happens in Parkinson’s disease) and following L-DOPA replacement therapy. Therefore, it has been suggested that endocannabinoid system modulation may constitute an important component in new therapeutic approaches to the treatment of motor disturbances. In this article we will review studies supporting the endocannabinoid modulation of dopaminergic motor circuits.

The discovery and the following investigation of the endocannabinoid system have demonstrated its implication in a large variety of functions such as regulation of appetite and energy metabolism, pain and inflammation, neuroprotection, and motor control. The endocannabinoid system is also a modulator of the basal ganglia circuitry functionality (Benarroch, 2007; Fernandez-Ruiz, 2009) and therefore, it may be considered as a potential pharmacological target for the treatment of movement disorders. This review is focused on the endocannabinoid modulation of dopaminergic motor circuits.

The cannabinoid receptors, endocannabinoids and the proteins for their biosynthesis and degradation constitute the key components of the endocannabinoid system (Di Marzo et al., 1998). CB1 receptors and endocannabinoids are highly expressed in the basal ganglia and have close connections with the dopaminergic system, being involved in the central regulation of motor functions.

Neurons expressing D1 receptors form the direct pathway, which projects to the GPi and the SNpr, while neurons expressing D2 receptors constitute the indirect pathway, projecting to the GPe (Figure 1) (Paul et al., 1992; O’Connor, 1998; Nicola et al., 2000; Onn et al., 2000; Svenningsson et al., 2000). A potential interaction between the D1/D2 and CB1 receptors at the level on the G-protein/adenylyl cyclase signal transduction mechanism has been suggested (Giuffrida et al., 1999; Meschler and Howlett, 2001). Combined activation of CB1 and D1 receptors results in a net decrease in adenylyl cyclase, a subsequent decrease in the inhibitory activity of direct striatal projection neurons and finally a decreased motor response due to enhanced activation of SNpr neurons. Conversely, activation of CB1 and D2 receptors together stimulates adenylyl cyclase (Glass and Felder, 1997), potentiating the indirect striatal pathway neurons that in turn activate neurons of the STN, also resulting in motor inhibition (Brotchie, 2003; van der Stelt and Di Marzo, 2003). These data indicate that endocannabinoid system acting on striatal CB1 receptors play a significant role in the regulation of basal ganglia motor circuits. Although CB1/D2 receptor heterodimerization has been demonstrated in transfected cells by co-immunoprecipitation and Forster Resonance Energy Transfer (FRET) techniques (Kearn et al., 2005; Marcellino et al., 2008), functionality of those heteromers in striatal glutamatergic terminals is not supported by recent studies (Kreitzer and Malenka, 2007).

In the last years, the transient receptor potential vanilloid type 1 (TRPV1) has gained attention for its ability to bind cannabinoids. Although neuronal expression and functionality of TRPV1 channels are controversial (Mezey et al., 2000; Cristino et al., 2006; Cavanaugh et al., 2011), this receptor is present in the basal ganglia. Indeed, TRPV1 is located on nigrostriatal terminals and on tyrosine hydroxylase positive cells in the substantia nigra pars compacta (SNpc) (Mezey et al., 2000; Micale et al., 2009) which makes it a good candidate for directly modulating dopaminergic neurotransmission (Figure 1). On the other hand, the orphan G-protein-coupled receptor 55 (GPR55) has been identified as another possible cannabinoid receptor (Ryberg et al., 2007) that, in contrast to classical CB1 and CB2, is coupled to Gq, Gα12 and Gα13 proteins (Sharir and Abood, 2010). Despite its high expression in the striatum (Sawzdargo et al., 1999), conflicting pharmacological findings make difficult to consider the GPR55 as a novel cannabinoid receptor (Oka et al., 2007; Lauckner et al., 2008; Sharir and Abood, 2010). Future investigations will clarify the role of TRPV1 and GPR55 in modulating basal ganglia circuits.

Functional Interactions between Endocannabinoid and Dopaminergic Systems in the Basal Ganglia

In accordance with its neuroanatomical distribution, functional and behavioral studies have suggested that the endocannabinoid system can act as an indirect modulator of dopaminergic neurotransmission in the basal ganglia.

It has been hypothesized that the inhibition of motor behavior mediated by cannabinoids could be related to a reduction in dopaminergic circuitry activity. Rotational studies in rats receiving local injections of cannabinoid compounds into the basal ganglia suggest that dopamine-cannabinoid interaction is not a direct mechanism. For instance, cannabinoids increase or decrease motor behavior when locally administered into the direct (Sanudo-Pena et al., 1996, 1998) or indirect pathway, respectively (Sanudo-Pena and Walker, 1997; Miller et al., 1998). Neuroanatomical studies showing that CB1 receptors are not present on dopaminergic neurons or terminals (Julian et al., 2003; Matyas et al., 2006) suggest that CB1-mediated modifications of nigrostriatal dopaminergic circuits are exerted indirectly by modulation of inhibitory or excitatory inputs to the midbrain dopamine neurons. Indeed, cannabinoids are known to dampen both glutamate and GABA transmission in the basal ganglia (Szabo et al., 2000; Gerdeman and Lovinger, 2001; Wallmichrath and Szabo, 2002).

The increased activity of SNpc neurons observed after CB1 receptor activation is in agreement with in vivo microdialysis experiments showing enhanced dopamine release in the striatum after exogenous or endogenous cannabinoid agonists administration (Tanda et al., 1997; Solinas et al., 2006). However, this effect is not mediated locally at the terminal level, but rather involves changes in the firing activity of SNpc neurons, since in vitro studies in striatal slices have shown that CB1 activation has no effect on dopamine release (Kofalvi et al., 2005). Contrary to CB1-mediated mechanisms, the effects of endocannabinoids on dopamine transmission may be mediated by direct mechanisms. Indeed, the endocannabinoid anandamide and some analogs (but not classic cannabinoid as Δ9-THC), acting via postsynaptic TRPV1 receptors, may reduce nigrostriatal dopaminergic cell activity (de Lago et al., 2004). However, other authors have reported an increase of dopamine release after activation of TRPV1 receptors in the SNpc (Marinelli et al., 2003, 2007), although this enhancement may be mediated by TRPV1 receptors located in glutamatergic terminals in the SNpc rather than by receptors located in dopaminergic terminals.

Pathological Implications of the Interaction between Dopamine and the Endocannabinoid System

As described above, neuroanatomical studies have located cannabinoid receptors in the basal ganglia, and it is widely accepted that the endocannabinoid system influence physiological motor function. These facts predict that pharmacological modulation of the endocannabinoid system may also be beneficial under pathological conditions pertaining to decreased dopamine signaling or the chronic treatment with L-DOPA.

Studies in animal models and patients of PD have indicated that dopaminergic neuronal degeneration produces an imbalance between the direct and the indirect basal ganglia pathways. This imbalance is manifested as reduced activity of striatal GABAergic neurons in the direct pathway and hyperactivity in the indirect pathway striatal neurons. Moreover, glutamatergic input from the cortex to the striatum is augmented after dopaminergic denervation (Tang et al., 2001; Tseng et al., 2001; Gubellini et al., 2002; Mallet et al., 2006). Within the basal ganglia, CB1 receptors are principally expressed on presynaptic cortical glutamatergic terminals and presynaptic striatal GABAergic terminals (Benarroch, 2007). The activation of CB1 receptors reduces the glutamate release from the cortex to the striatum (Gerdeman and Lovinger, 2001; Gubellini et al., 2002; Brown et al., 2003) and GABA release to the SNpr (Wallmichrath and Szabo, 2002). In addition, endocannabinoids and CB1 receptors play an important physiological role in the long- and short-term regulation of the synaptic transmission in the basal ganglia shaping the striatal output and therefore modulating motor activities. The two classic forms of long-term synaptic plasticity, long-term potentiation and long-term depression (LTD) are expressed at corticostriatal synapses and abolished in animal models of PD (Centonze et al., 1999; Picconi et al., 2005; Calabresi et al., 2007; Kreitzer and Malenka, 2007). Using different LTD induction paradigms it has been described that this form of plasticity is mostly controlled by endocannabinoids (Shen et al., 2008; Lovinger, 2010). Although probably other mechanisms are also involved in the dopaminergic control of striatal plasticity, pharmacological manipulation of the endocannabinoid system under parkinsonian conditions has been proved not only to rescue LTD in striatum but also to improve the motor deficits evident after dopaminergic denervation (Kreitzer and Malenka, 2007).

In line with this, behavioral studies have shown that modulation of the endocannabinoid system can have a therapeutic impact in animal models of PD. Behavioral changes caused by the induction of parkinsonism in rats have been improved by the administration of CB1 receptor antagonists both in unilateral and bilateral PD models in rodents (Fernandez-Espejo et al., 2005; Gonzalez et al., 2006; Kelsey et al., 2009). In MPTP-lesioned marmosets, CB1 antagonist administration increased locomotor activity but failed to improve bradykinesia or posture (van der Stelt et al., 2005). On the other hand, co-administration of L-DOPA with CB1 antagonists added a positive improvement of the motor symptoms assigned to the antiparkinsonian drug in parkinsonian animals (Kelsey et al., 2009). The latter data suggest that combined therapy with antiparkinsonian drugs and cannabinoid antagonists may permit a reduction of L-DOPA dose and therefore, delay the emergence of the motor side effects induced by the chronic treatment with L-DOPA.

Implication of the Cannabinoid System in L-DOPA Induced Dyskinesia

The emerging role of the endocannabinoid system as modulator of neurotransmission in the basal ganglia identifies it as a potential pharmacological target for treating motor complications derived from the chronic treatment with L-DOPA. L-DOPA induced dyskinesia (LID) constitute one of the most disabling complications derived from the long-term therapy with L-DOPA affecting up to 40% of PD patients after 5 years of treatment (Ahlskog and Muenter, 2001). Cannabinoid agonists could exert antidyskinetic effect by regulating glutamatergic release in the striatum and/or by re-establishing endocannabinoid-mediated synaptic plasticity affected by dopaminergic denervation. In this sense, pharmacological agents with antidyskinetic properties such as serotonergic 5-HT1B agonists are able to ameliorate the motor complications by depressing the glutamatergic corticostriatal transmission (Mathur et al., 2011).

Taken together, changes in the cannabinoid system are observed after dopaminergic denervation and manipulation of this system has proved to have beneficial effects on parkinsonian symptoms in animal models and PD patients. However, the putative role of cannabinoids in LID is still a matter of controversy. The complex localization of the cannabinoid receptors at different sites in the basal ganglia circuits may contribute to the paradoxical observed effects. Further investigations are needed to clarify the role of the cannabinoid system in LID.

In conclusion, the endocannabinoid system modulates nigrostriatal dopamine transmission both via direct and indirect mechanisms. This system has an important role in dopamine-related movement disorders, as PD, and represents a framework for novel therapeutic approaches in the future.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to acknowledge Jan Tonnesen for careful reading and pertinent comments. Financial support UFI 11/32.